Recently, SSC is noted as a new communication system. In SSC, a SAW element is important as a component of an arrangement mainly utilizing a correlating function in the receiver, namely as a component of a SAW convolver.
Heretofore, bi-phase shift keying has often been used as a modulation system in SSC. However, MSK (minimum shift keying) is also noted as a digital signal modulation system.
In comparison with BPSK waves, MSK waves have a narrower band with respect to the same clock, have a better energy concentration to the main lobe, and have a better off-band characteristic. More specifically, under a limited band width for a system, a higher clock frequency is permitted in MSK than in BPSK, and a disturbance to adjacent channels can be diminished.
A problem with the MSK system is that generation of MSK-modulated waves was relatively complicated while BPSK-modulated waves were readily obtained.
Properties of MSK waves and MSK-wave generating processes from the viewpoint of circuitry are described in:
(Document 1)
"Minimum Shift Keying: Spectrally Efficient Modulation", S. Pasupathy, IEEE Communications Magazine, July 1979, pp.14-22.
(Document 2)
MSK generating processes using SAW elements are described in:
"An Application of SAW Convolvers to High Bandwidth Spread Spectrum Communications", J. H. Goll, D.C. Malocha, IEEE Transactions on Microwave Theory and Techniques, vol. MTT-29, No. 5, May 1981, pp.473-483.
(Document 3)
"Surface-Acoustic-Wave MSK Matched Filter and its Application", Junichi Tominaga, Shibayama, Transactions on Electronic Communication Academy, '85/12, Vol. J68-C, No. 12, pp.1044-1052.
(Document 4)
"SAW Filters for CPSM Spread Spectrum Communications", W. R. Smith, 1977 Ultrasonics Symposium Proceedings, pp.524-528.
According to Document 1, MSK waves may be expressed ##EQU1## where
a.sub.I (t): even number bit of a code
a.sub.Q (t): odd number bit of the code
T: clock period
f.sub.c : center frequency
Document 1 teaches a method of establishing Equation (1) by a circuit arrangement, and this requires a much complicated structure.
Document 2 and Document 3 teach methods which require an impulse generator. In this respect, Document 4 teaches a method having a very simple structure.
The theory of the MSK-wave generating filter (called "MSK filter") of Document 4 is explained below, referring to FIG. 13.
In FIG. 13, E refers to a baseband code, T refers to a clock period, M refers to a mixer, F refers to a signal source of center frequency f.sub.1, B refers to an MSK filter, A refers to a piezoelectric substrate, C refers to an MSK filter input (BPSK), T.sub.1 refers to an input transducer, T.sub.2 refers to an output transducer, D refers to an MSK filter output, and G refers to a SAW absorber.
The MSK filter input C is obtained by mixing the baseband signal E of clock period T with an output of the sine wave signal source F of center frequency f.sub.1. As a result, the input signal C behaves as a BPSK wave. The MSK filter consists of the input transducer T.sub.1 and the output transducer T.sub.2 obtained by providing aluminum or other metal on the piezoelectric substrate made from crystal, lithium niobate or other material. The SAW absorber G is provided at each end of the piezoelectric substrate for each input or output transducer T.sub.1 or T.sub.2 to absorb undesired SAW's.
The input transducer T.sub.1 has a sufficiently small number of electrode fingers so as to establish a wideband characteristic.
The output transducer T.sub.2 has the electrode length of time width T and the center frequency f.sub.2, and satisfies the following relationship (see Document 4): ##EQU2## where f.sub.c and T are the same values as Equation (1).
When an input BPSK electric signal C is entered in the MSK filter, it is converted into a SAW by the input transducer T.sub.1 and travels to the right (H). Thereby, an output electric signal D is obtained from the output transducer T.sub.2.
When equations (2), (3) and (4) are satisfied, it is noted that the output wave D exhibits the MSK wave shown by Equation (1). Here, it is important that the input transducer T.sub.1 has a wideband characteristic as described above and that the output transducer T.sub.2 has the time width T and the center frequency f.sub.2. Because of these characteristics, the output is the convolution between the input BPSK wave C and the output transducer T.sub.2 and satisfies Equation (1).
As described, the MSK filter is advantageous in its simple structure.
However, when using MSK modulation in SSC, the receiver requires a larger space and a higher cost than the use of BPSK as shown in FIG. 14 because MSK is used in combination with a SAW convolver in the receiver side. In FIG. 14, a received signal in SSC labeled with I is in the form of MSK modulation. J refers to a SAW convolver which is an element for effecting a convolution computation between the entered signal I and the reference signal D. The reference signal D is an MSK-modulated wave as shown in FIG. 13.
In BPSK, the entered signal I is a BPSK wave, and the reference signal of FIG. 14 may be BPSK. Therefore, the MSK filter B of FIG. 14 may be omitted. However, MSK necessarily requires the MSK filter B and this increases the space and the cost.